Present day lignite mining techniques include, in addition to overburden removal, the removal of about four (4) inches of the top of a vein and a leaving of about the bottom four (4) inches of the vein to reduce the clay, rock and sand content, commonly referred to as gangue, mixed with these two portions of the vein. Such practice virtually eliminates the production of lignite from thin veins and results in significant losses of lignite in the two four (4) inch throwaways.
Apparently little effort has been expended in studying means to upgrade lignite. Power plants which use lignite are typically located near the mine site and use run-of-mine (ROM) center cut lignite containing the entrained gangue and ash producing constituents yielding a low overall BTU content and considerable ash disposal. DOE has published the results of a washability study in PMTC-5(79) and several companies have allegedly made a washability study, but none has reported any effort to scale up a plant to use the data obtained in these studies.
Several studies, reported in the literature, relative to the potential for pipeline transport of lignite have noted that the ROM lignite ground to a size to be so transported undergoes a hydrodegradation during pumping studies.
One such study was conducted by the Bureau of Mines, (Bureau of Mines Report of Investigation 5404, 1958), the equipment used in this study was modified from that used in the ASTM jar-tumbler test for coal..sup.1 The jars were 8-inch lengths of 8-inch pipe with 3 full-length, equally spaced, 1-inch-high longitudinal lifters welded to the inside. As in the ASTM tests, the jars were rotated on horizontal, longitudinal axes at 40 r.p.m. Individual tests were conducted by placing 500 grams of lignite and 1,000 cc of water in a jar, tumbling it for definite periods of time (7 to 64 hours), and determining the size analysis of the tumbled slurry. Size analysis was determined by wet screening the slurry over 4-, 8-, 16-, 30-, and 50-mesh screens and determining the size consist of the minus-50-mesh fraction in a Palo-Travis particle-size apparatus. Degradation was obtained by comparing the size analysis of the tumbled slurry with the similarly determined size analysis of an untumbled sample of the same material. FNT .sup.1 American Society for Testing Materials, Standard Method of Tumbler Test for Coal: ASTM Standards, D-441-45, pt. 5, 1949, pp. 629-632.
Two series of jar-tumbler tests were run on Sandow (Texas) lignite--one on a 1/8- by 0-inch and the other on the 1/4- by 0-inch size ranges. The report noted that both sizes exhibited a considerable amount of degradation, characterized by the formation of large amounts of ultrafine material (5 to 10 microns). In these tests the 1/4- by 0-inch lignite apparently suffered less degradation than did the 1/8- by 0-inch; the 1/4- by 0-inch had a slightly greater size stability and significantly lower increases in percentage of minus -200-mesh material.
To determine the variability in degradation characteristics of different lignites, a series of additional jar-tumbler tests was run with 4 Texas lignites--each in 3 size ranges (1/8- by 0-, 1/4- by 0-, an 1/2- by 0-inch). These tests were less comprehensive than the tumbler tests on the Sandow lignite, and only the amounts of minus-50-mesh material were determined. The test results showed significant differences in the rates of generating minus-50-mesh material of the various lignites and of different size ranges of the same lignite. For the 3 size ranges, the average rate of increase in minus-50-mesh material of the Titus County lignite was roughly 1.7 times that of the Rusk County lignite. It is interesting to note that the ratios between the hourly rates of increase in percentages of minus-50-mesh material of the 3 size ranges were relatively constant; the hourly increases in percentage of minus-50-mesh material by the 1/8- by 0-and 1/2- by 0-inch sizes averaged, respectively, 0.564 and 1.244 times that of the 1/4- by 0-inch sizes. Such data indicates there is significant hydrodegradation on contact of the lignite with water in jar tumbler tests.
THus, it was unexpected that lignite could be treated with water and the lignite separated from the gangue in an economical manner which permitted use of the normal "throwaway" portion of a vein and even the ability to mine thin veins, which carried some overburden and underburden, to upgrade the lignite and in many instances produce a pumpable slurry.